If its casual play then call it however the heck you want. A tournament or league though (at least I would think) this sort of thing ought to be address by the TD/LD especially if the water level varies. Personally, I'd be tempted to make the entire canal OB outlined with white paint. Of course I am saying all this site unseen. I do not recall seeing anything in the rules having to do with the width of the stream.
https://www.pdga.com/rules/official-rules-disc-golf/80603
https://www.pdga.com/rules/official-rules-disc-golf/80602
In essence, the way I treat it is, if it isn't defined as being OB then it isn't. Most folks just assume that permanent standing water such as lakes, ponds, rivers or creeks but if there isn't a TD, LD or sign to indicate what is and isn't OB then it's all inbounds. Am I right in this line of thought?
In any case, sometimes to eliminate bad feelings in casual rounds I like to always address the OB on each hole before anyone tees off. "You play that roadway as OB?" "Yep" kind of thing goes a long way to fostering a more pleasant round.
-Dave